What had begun as largely peaceful demonstrations followed the acquittal of a white police officer in the fatal shooting of a black man.

Several hundred people gathered near Washington University after dark as the main rallies and marches wound down on Saturday evening.

Police marched on protesters after they refused to leave the centre of St Louis. Source: Reuters

Chopper cams captured the police marching over the woman in the crowd. Source: Fox2

As the night wore on violence broke out between some of roughly 100 remaining protesters, some holding bats or hammers, and riot-clad police who ordered them to disperse.

Police declared the scene an unlawful assembly and threatened to deploy tear gas and arrested at least eight people.

"We had been getting such a good turn out earlier and it was a peaceful protest," said Jomar Jackson, 32.

The woman was mowed down by the marching police force. Source: Reuters

"But then a bunch of people came and decided to be disruptive."

Elisheva Heit, whose flower shop window was smashed on the eve of its grand opening, said "I don't understand how this would bring the poor guy back to life."

Sweeping up glass, she asked passing officers "This is how you protect people?"

The riot police walked over the woman as they cleared the streets. Source: Fox2

The demonstrations began peacefully on Friday after Circuit Judge Timothy Wilson acquitted former St. Louis Police Officer Jason Stockley, 36, of first-degree murder in the shooting death of Anthony Lamar Smith, 24.

Chopper cam video from Fox2 shows the heavily armoured riot police march on the crowd, storming over the top of an elderly woman in the crowd.

But as on Saturday, Friday's protests ended in late night violence, with 33 arrests after clashes on which 10 officers were hurt.

Source: Fox2

"We don't want to see property destruction or see people getting hurt," Elad Gross, a 29-year-old St Louis civil rights attorney, told Reuters as activists gathered peacefully at another protest site in a park on Saturday.

"But this is a protest that addresses injustices not only happening here in St. Louis but around the country."

Rock band U2 cancelled a concert scheduled for Saturday in St. Louis, and singer Ed Sheeran did the same for his show on Sunday, citing security concerns.